The present invention relates to variable ratio transmission and, in particular, it concerns a continuously variable transmission based on a conical arrangement of gear wheels.
Most conventional transmissions provide a relatively small number of discrete gear ratios between the rates of rotation of an input shaft and an output shaft. For example, in automotive applications, most vehicle gear boxes offer 3, 4 or at most 5 forward gear ratios to match road speed to a suitable engine speed. Although this number of gears is sufficient to bring the engine speed within its normal operative range, the matching of engine speed to road speed and desired power is typically very inefficient. For any given desired power output, an engine should optimally be run at a corresponding given engine speed. However, due to the very limited ratios available, the engine speed will almost always be either too low, resulting in less power than desired, or too high, resulting in energy wastage.
Theoretically, the engine speed mismatch could be reduced by providing more numerous, more closely spaced gear ratios. Since, however, each gear ratio is defined by the ratio of the numbers of gear teeth on two meshed gear wheels, each change of ratio requires disengagement of a first pair of gear wheels and engagement of another. Additional disengagement and slow re-engagement of a clutch is also required to allow matching of the new engine speed. As a result, the introduction of more frequent ratio changes would itself be highly disruptive to the operation of the transmission, causing corresponding lowering of efficiency.
In an attempt to improve efficiency and/or performance, various systems have been developed to provide a continuously variable transmission ratio, commonly referred to as "continuously variable transmissions" (CVT). These systems have abandoned the intermeshed gear wheel structure in favor of traction-based engagement which allows smooth ratio transitions without disengagement. Among the most prevalent examples of such systems are the Van-Doome belt or "V-belt", and the toroidal traction drive.
Theoretically, because of correct matching of engine speed to the required power, CVT systems should be considerably more efficient than conventional fixed ratio transmissions. In practice, however, energy savings are small to non-existent. This is primarily due to major frictional losses and slippage which are generally inherent to traction-based systems.
There is therefore a need for a variable transmission based upon intermeshing gear wheels which would offer transition between different gear ratios without disrupting the driving engagement. Such a transmission would allow substantially unlimited numbers of gear wheels, thereby approaching near-optimal use of engine performance.